The present invention relates generally to integrated circuits and, more particularly, to state retention power gated (SRPG) cells used in integrated circuits.
Lower power consumption has been gaining importance in integrated circuit data processing systems due to, for example, wide spread use of portable and handheld devices. Most circuits in handheld devices are typically off (e.g., in an idle or deep sleep mode) for a significant portion of time, consuming only leakage power. As transistor leakage currents increase with finer geometry manufacturing processes, it becomes more difficult to meet chip leakage targets using traditional power reduction techniques. Therefore, reducing leakage current is becoming an increasingly important factor in extending battery life.
One method that has been used to reduce leakage current is to “power gate”, or cut off power for certain blocks of the integrated circuit that are not needed when the device, such as when the device is in a low power or sleep mode. However, in doing so, the state of the circuit is lost if a means is not provided to retain the state during the powered down mode. Therefore, a SRPG cell is provided which has at least one separate latch to retain the logic state of the cell during power down mode.
In low power designs, there are two classic kinds of SRPG cells: clock state independent (CSI) SRPG cells and non-CSI SRPG cells. Non-CSI SRPG cells are smaller in size and consume less power than CSI SRPG cells, but have specific clock state requirements. For positive edge triggered non-CSI SRPG cells, the clock needs to be held low during the assertion/de-assertion of a power gating signal, while for negative edge triggered non-CSI SRPG cells, the clock needs to be held high during the assertion/de-assertion of a power gating signal.
Therefore, in existing designs, only flip-flops that meet the above non-CSI SRPG clock state requirements are replaced with non-CSI SRPG cells. The others have to be replaced with CSI SRPG cells, which are large in size and consume more power.
Therefore it would be desirable to improve the SRPG cells to solve the above problems.